The need for increased fuel economy and improved pollution control have caused designers of internal combustion engines to seek substantially improved fuel supply systems. In response, unit fuel injectors providing precise, reliable and independent control over injector timing and metering have been developed and are widely used.
Fuel injectors of the "unit" type, such as shown in U.S. Pat. No. 4,471,909 to Perr, have a nozzle and a reciprocating injection plunger mechanically actuated by an engine camshaft to force fuel from the nozzle. The camshaft produces an advancing or downward force on the injector plunger (the direction toward the combustion chamber will be referred to herein as advanced or downward, although the injectors could be mounted with any physical orientation).
While effective for their intended purposes, such unit injectors were only designed to achieve injection pressures in the range of 15,000 to 20,000 psi. This range is not necessarily sufficient to achieve the high performance, low fuel consumption, and minimal pollution demanded of modern diesel engines.
In response to this increased performance requirement, very high pressure unit injectors have been designed. One such type of unit injector is shown in the commonly assigned U.S. Pat. Nos. 4,986,472 to Warlick et al. and 4,721,247 to Perr. These injectors have an injector housing with a plunger assembly disposed within a central axial bore. The plunger assembly includes a lower plunger, an intermediate plunger, and an upper plunger. The lower plunger reciprocates within the central bore to cause a variable quantity of fuel to be first metered and then subsequently injected into the engine during downward portions of the reciprocating motion of the plunger assembly. A timing chamber formed between the upper and intermediate plungers receives timing fluid during the metering phase of the injection cycle so as to create a variable length hydraulic link between the upper and intermediate plungers. The amount of timing fluid can be adjusted to vary the timing of the fuel injection for enhanced engine operation. Injectors of this general type can be manufactured either as "open nozzle" unit fuel injectors or as "closed-nozzle" types having a pressure-operated valve mechanism for opening and closing the injector spray holes.
In high-pressure unit injectors, SAC pressures (the pressure of the fuel in the injection chamber just above the injector spray holes) as high as 30,000 psi or more are desired to assure complete fuel atomization. When an injector is designed to achieve very high SAC pressures at low engine speeds, there arises a danger that excessive loads will develop in the injector drive train. To control the upper limit of injection pressure, a pressure responsive valve such as disclosed in the '247 or '472 patents may be provided to drain timing fluid during downward movement of the upper plunger. Provision of a pressure-limiting valve allows the unit injector and associated camactuated drive train to be designed to provide extremely high injection pressures, for example up to 30,000 psi, even at low engine speed, without risking excessive pressure at higher engine speeds. The timing fluid which remains in the timing chamber as the lower plunger reaches its lowermost position must, of course, be discharged. It is desirable to "throttle" this discharge, however, to create a high "blowdown load" or pressure on the lower plunger to prevent the lower plunger from bouncing back at the end of its travel. Such undesirable bouncing of the lower plunger can cause secondary end-of-injection "dribbling of fuel" which can cause insufficient fuel combustion, pollution, and poor fuel economy.
To achieve a desired degree of blowdown load, a pressure release valve such as disclosed in U.S. Pat. No. 4,249,499 has been provided, adapted to operate at the termination of injection. Such a valve would need, however, to have a different operating characteristic from that of the pressure relief valve of the '247 and '472 patents, which have valves designed to operate during the fuel injection event, i.e. where the lower plunger has yet to reach its lowermost position. Two such pressure relief valves, having different operating characteristics, would necessarily conflict.
This dilemma was partially solved in the '472 patent by providing a spillport located axially in the injector body so as to communicate with the timing chamber only when the lower plunger has very nearly reached its lowermost position. Careful control over the position and size of the spillport, plus control over the shape of the plunger profile which controls flow through the spillport (see FIGS. 14(a)-14(c) and columns 14-16 of the '472 patent) can result in highly desirable end-of-injection blowdown loads. As shown in the drawings of this patent, the pressure within the timing chamber of this type of injector would inherently be nonsymmetrical with respect to the injector central axis during blowdown. This lack of symmetry could lead to uneven wear.
Inherently, however, pressure produced by prior injector designs during blowdown is a function of engine speed. At low engine speeds in particular, this reduction in blowdown pressure may still permit an undesired upward movement of the lower plunger after injection of the metered fuel has been completed. This upward movement results in secondary injection, which is the leakage of additional, undesired fuel into the combustion chamber following termination of the main injection event. Because this secondary injection fuel is introduced too late in the combustion cycle to be burned effectively, it enters the engine exhaust stream as an incompletely combusted hydrocarbon component. This results in vehicle emissions characteristics that are undesirable in environmental terms. Also, this secondary injection fuel performs no useful work; thus, the secondary injection phenomenon results in poorer fuel economy than could be attained with a clean cutoff of injection.
The problem of secondary injection has sometimes been addressed by continuing to move the upper injector plunger downwardly after the upper plunger physically contacts the lower plunger and the lower plunger reaches its nominal end-of-travel position. In such injector systems, the lower part of the plunger .is actually "overdriven" to resiliently compress the injector drive train and temporarily deform the injector housing. The use of such large overtravel forces results in increased wear and fatigue effects on system components. As a result, injector components must be constructed more ruggedly and to closer manufacturing tolerances than would otherwise be needed, and the costs of manufacturing and repairing the injection system are increased.
Despite notable advancements achieved before, it has not been possible to produce a minimum-cost, highly reliable, hydraulically variably timed unit injector which form a blowdown pressure sufficient over a broad range of engine speeds to prevent secondary injection. Therefore, there is a need for a novel and improved unit injector design which meets these criteria.